Lord Peter Mandelson has been asked to testify to the US Congress over his relationship with the paedophile financier Jeffrey Epstein.
A letter from two Democrat members of the House of Representatives called on the former British ambassador to Washington to answer questions from the House Oversight Committee as part of their probe into Epstein.
The disgraced peer's links with Epstein have led to calls for Sir Keir Starmer to stand down as prime minister.
The prime minister's judgement has been questioned by critics over his appointment of Lord Mandelson as US ambassador, a role considered the most prestigious posting in British diplomacy.
The letter to Lord Mandelson, signed by representatives Robert Garcia and Suhas Subramanyam, said: "While you no longer serve as British ambassador to the United States and have stepped down from the House of Lords, it is clear that you possessed extensive social and business ties to Jeffrey Epstein and hold critical information pertaining to our investigation of Epstein's operations.
"Given the appalling allegations regarding Epstein's conduct, we request that you make yourself available for a transcribed interview with Committee staff regarding the crimes of Jeffrey Epstein and his co-conspirators."
The letter added: "Numerous pieces of evidence have come to light demonstrating your close ties to Jeffrey Epstein over the span of multiple years."
Among the evidence, it listed Lord Mandelson's "handwritten note" in a 50th birthday book for Epstein in which he described him as his "best pal".
It also highlighted the peer's stay at Epstein's New York apartment in 2009 while Epstein was serving an 18-month sentence for soliciting prostitution and soliciting a minor.
Also cited were photographs of Lord Mandelson in his underwear in the latest Department of Justice release from the Epstein files.
The letter gave Lord Mandelson a deadline of 27 February to respond to the two US politicians "due to the urgency and gravity of this matter".
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Lord Mandelson served as US ambassador between February and September last year before he was sacked by Sir Keir over his relationship with Epstein.
Lord Mandelson has repeatedly denied all wrongdoing.
He previously apologised for maintaining his relationship with Epstein, which he admitted went on for too long.
Last year, the two US representatives made a similar request to Andrew Mountbatten-Windsor, who was then a prince.
(c) Sky News 2026: Lord Mandelson asked to testify in US Jeffrey Epstein investigation
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